The present invention is directed to a catalyst composition, its method of preparation, and a process for using the composition for alcohol oxidation to the corresponding aldehydes.
The surface metal oxide species on oxide supports play a crucial role in the catalytic processes of supported metal oxide catalysts, which have been widely used as catalysts in numerous industrial applications: MoO3/gxe2x80x94Al2O3 and WO3/gxe2x80x94Al2O3 catalysts for hydrodesulfurization (HDS) and hydrodenitrogenation (HDN),1-2 V2O5/TiO2 catalysts for o-xylene oxidation to phthalic anhydrid3,4 and selective catalytic reduction (SCR) of NOx5. The industrial development of supported metal oxide catalysts over the past five decades has been summarized in a recent review paper.6 
Fundamental information about the surface metal oxide molecular structures have been obtained by a battery of physical and chemical techniques, including Raman spectroscopy, infrared spectroscopy (IR), X-ray photoelectron spectroscopy (XPS), UV diffuse reflectance spectroscopy (UV-vis), solid-state nuclear magnetic resonance (NMR), extended X-ray absorption fine structure (EXAFS), Mossbauer spectroscopy, surface acidity, adsorption, and probe reactions.7-10 The reactivity of the surface metal oxide species in various supported metal oxide catalysts has been probed by different chemical reactions including methanol oxidation, alkane oxidation, SO2 oxidation, and the selective catalytic reduction of NOx.6 Correlation of the catalytic reactivity with the corresponding molecular structural information about the surface metal oxide species has elucidated many fundamental issues about the catalytic properties of the surface metal oxide species during catalytic reactions: the roles of terminal double bonds, bridging bonds, adjacent or neighboring sites, secondary metal oxide additives, support ligands, and preparation methods. The fundamental information obtained from these molecular structure-reactivity relationships has great potential for the molecular design and engineering of supported metal oxide catalysts for various catalytic applications.
The formation of a two-dimensional metal oxide species on surfaces of oxide supports through thermal spreading of three-dimensional bulk metal oxides (schematically represented in FIG. 1) is well documented in the catalysis literature.7,8 Thermal spreading is a spontaneous process from a thermodynamics perspective. However, its kinetics are constrained and require a high temperature for surface diffusion or migration of one metal oxide component over the surface of a secondary oxide support to occur at an appreciable rate. In the context of thermal spreading, Tammann temperature (TTam greater than  greater than 0.5Tmp; Tmp=bulk melting point of the dispersed metal oxide) is often used to estimate the temperature for thermal treatments. The driving force for thermal spreading and formation of the surface metal oxide monolayer is a concentration gradient of the dispersed component or a decrease in the overall system surface free energy.
In contrast, little information is available on the spreading of metal oxides over oxide supports during catalytic reactions. Gasior et al.11 previously reported the spreading of vanadia over the surface of TiO2 (anatase) grains in a V2O5 and TiO2 (anatase) physical mixture during o-xylene oxidation at 360xc2x0 C., which was manifested by increase in both conversion and phthalic anhydride selectivity with reaction time. Cavalli et al. earlier observed that soluble bulk V2O5 spreads over the free rutile surface and, on the contrary, the insoluble vanadium oxide partly segregates and forms the soluble bulk V2O5, during ammoxidation of toluene to benzonitrile at 320-390xc2x0 C.12 However, the reaction temperatures (xcx9c360xc2x0 C.) were much higher than the Tammann temperature (210xc2x0 C.) of crystalline V2O5, implying that thermal spreading might have dominated the spreading kinetics during the reaction.
It has now been discovered that catalysts useful for producing aldehydes from alcohols contain a physical mixture comprising: (a) a particulate source of a metal oxide support, and (b) a source of metal oxidation catalyst selected from the group consisting of oxides of group VA, VIA, and VIIA metals that are catalytically active for alcohol conversion to aldehydes. During the course of the conversion reaction, a catalytically active monolayer of the VA, VIA, or VIIA metal oxides migrates to surfaces of the metal oxide support particles and form supported, catalytically active compositions with enhanced selectivity and activity for the conversion reaction.
Catalysts formed by the present invention and the resulting alcohol conversion process are comparable to prior art catalysts formed via the more expensive alkoxide impregnation manufacturing methods of the prior art. These and other advantages will be readily recognized by those of skill in the art.
A method for forming a catalytically active composition in accordance with the invention comprises:
forming a fixed bed of solids comprising an intimately admixed composition comprising a particulate source of metal oxide support, and a particulate source of catalytically active metal oxide selected from metal oxides from groups VA, VIA, and VIIA, and
passing a reactant stream containing an alcohol and oxygen through said fixed bed at conditions suitable for conversion of said alcohol to a corresponding aldehyde.